1. Field of the Invention
The present invention relates to a drainage pump and, a hydraulic turbine, and more particularly, to a bearing unit exhibiting excellent wear resistance, satisfactory reliability and assembling facility, with the bearing unit being adapted to be utilized in a pump of a type adapted to be operated absent a clear water supply to the bearing portion thereof or a hydraulic turbine, and relates to a drainage pump and a hydraulic turbine each incorporating the bearing unit as well as a method for manufacturing the bearing unit.
2. Description of Related Art
Since substantially all precipitation flows in the drainage ditches due to the recent urbanization, the precipitation cannot be completely drained off and there is a tendency for municipal floods causing roads to be under water. Hence, drainage facilities, each provided with a drainage pump system, have been established, causing another problem that the cost of operation and maintenance increases. Therefore, an automatic operation drainage pump system has been examined. The drainage pump system of this type must improve the performance and reliability thereof and non-water-supply operation technology. In order to achieve this, an increase in capacity and precedent-standby operation have been studied.
The non-water-supply operation technology is to operate the drain pump while supplying no water to the space between a sleeve on a shaft and a bearing. Since no clear water supply apparatus is provided, a malfunction does not take place due to a failure of the clear water supply apparatus and that of a clear water supply sensor, resulting in satisfactory reliability. In order to enable the non-water-supply operation to be performed, a bearing unit must not be worn even if earth and sand contained in the drain invade thereto.
A conventional bearing unit adapted to the non-water-supply technology has been constituted by a sleeve (a sintered article) made of tungsten carbide (hereinafter "WC") exhibiting excellent wear resistance and corrosion resistance and by a ceramic bearing (a sintered article). Constructions fashioned of a sleeve made of WC and the ceramic bearing have been disclosed in JP-A-60-81517 and JP-A-60-88215.
The enlargement of the pump capacity inevitably requires the diameter of the bearing thereof to be enlarged. However, the conventional bearing unit fashioned of a WC sleeve and the ceramic bearing require a large-diameter sintered sleeve and a sintered bearing. However, a problem arises in that the sintering technology cannot meet the requirements and the weight of the parts cannot be reduced, resulting in the assembling of the bearing unit. Therefore, a hard film coating technology capable of coating a film having hardness equivalent to that of ceramics has been investigated. It is noted that a non-water supply drainage pump using a sleeve applied with a sprayed WC coating and a ceramic bearing has been disclosed, for example, in "Trybologist", No. 2, vol. 36, pp. 144 to 147.
Also the bearing for a hydraulic turbine must have wear resistance against water including sediment. Therefore, a conventional bearing for a hydraulic turbine has been made of ceramics such as silicon carbide (SiC).
As a general rule, a sprayed coating is used as the hard film for the bearing portion of a drain pump. However, the fact that the sprayed coating is inferior to a sintered article having the same composition in terms of the hardness and the strength causes an investigation for improving the hardness and the strength of the sprayed coating to be made. For example, technology for improving the adhesive strength and the strength by plasma-spraying a material composed of, by weight, 50% chromium and 50% nickel and by holding the sprayed coating at from about 700.degree. C. to 800.degree. C. for from about 1 to 100 hours has been disclosed in JP-A-57-2872. Another technology has been disclosed in JP-A-60-149762 which is capable of strengthening the sprayed coating by forming a 20 to 80 wt. % Ni-Cr sprayed coating, the main component of which is Ni-P on a ferrous material, and by subjecting them to a heat treatment set to from about 600.degree. C. to 1000.degree. C.
Since the hydraulic turbine has a large diameter, the conventional structure has an arrangement that sectioned ceramics are disposed. Therefore, there has been a desire of using the hard film similarly to the bearing for a drain pump. A bearing unit for a hydraulic turbine that uses a sprayed WC coating and chromium carbide (Cr.sub.3 C.sub.2) has been disclosed on pp. from 80 to 85, in resumes of the twenty-sixth lecture meeting titled "Turbo Machines", May 1991.
The precedent standby operation is an idle operation executed before commencement of drainage in a state where the water level is lower than the level at which the drainage operation can be performed. By performing the precedent standby operation, full power operation can be performed when the water level rises to a level at which the drainage can be performed. Therefore, a rapid increase in precipitation can be quickly accommodated with the subject system. However, the fact that the bearing must be idly performed without drain to be introduced into the sliding or bearing portion thereof for a short time causes the friction coefficient between the bearing and the sleeve must be extremely low so as to be freed from being damaged during the foregoing dry operation.
A bearing capable of performing both of a long time precedent standby operation, in which the idle operation is carried out for a long time, and the non-water-supply operation has not been developed yet. However, a method capable of supplying external water to the sliding portion of the bearing at the time of the idle operation has been proposed in JP-A-55-90718.
In order to drain water containing earth and sand, the bearing must have satisfactory wear resistance. In particular, the non-water supply bearing must have wear resistance against slurry because clear water for protecting the bearing and the sleeves is not supplied. The major portion of the earth and sand is composed of feldspar and quartz. Therefore, earth and sand have a maximum hardness of about Vickers (Hv)1000 (which is the hardness of quartz). Accordingly, the hardness required for the bearing and the sleeve is Hv 1000 or higher.
A sintered article composed one of WC, SiC and silicon nitride (Si.sub.3 N.sub.4) is excellent in the hardness while providing satisfactory wear resistance because these materials have a hardness of Hv 1000 or higher, that is, WC has a hardness of about Hv 1400, SiC has a hardness of about Hv 2800 and Si3N4 has a hardness of about Hv 1600. However, the sintering technology, the assembling facility and the manufacturing cost cause a limit to present in the size of the sleeve and the bearing that are fully made of a sintered material composed of WC, SiC or Si.sub.3 N.sub.4. Therefore, there has been investigated the use of a hard film in place of the aforesaid sintered article. Since the film thickness must be several hundreds microns if the quantity of wear is taken into consideration, there is a limit in the method of manufacturing the hard film. As the hard film for the bearing portion of a drainage pump, a thermally-sprayed coating mainly composed of WC or Cr.sub.3 C.sub.2 and having a thickness of from about 100 to 200 .mu.m has been widely used because a relatively thick film and satisfactory hardness can be obtained. However, the sprayed coating mainly composed of WC or Cr.sub.3 C.sub.2 generally has unsatisfactory hardness as compared with a sintered article although, depending upon the material, the forming method and conditions, with the aforesaid sprayed coating has a hardness of from about 600 Hv to 1000.
The relationship between a variety of conventional sprayed coatings and the wear resistance against water containing earth and sand will be described with reference to FIG. 21 which is a graph showing the wear rates measured by element experiments to be described later in the description about preferred embodiments. Each one of the sprayed coatings were formed on a stainless steel plate to serve as a rotary side specimen, while .alpha.- SiC was employed to serve as the stationary side specimen. The residual conditions were as follows: the surface pressure was 2 kg/cm.sup.2, the peripheral speed was 0.5 m/sec, the concentration of silica particles in earth and sand in water was 9 wt. %. It is noted that the detailed shape of the specimen and the sliding method and the like were the same as those of the experimental conditions shown in FIG. 19 with regard to the preferred embodiments.
The abscissa of a graph shown in FIG. 21 stands for cross-sectional Vickers hardness of the sprayed coatings, and the ordinate stands for relative wear ratio standardized while making the wear ratio of WC - 12% cobalt sintered article to be a reference. Sprayed coatings 1, 2 and 3 were conventional sprayed coatings formed by a high speed spraying method and a detonation spraying method and mainly composed of WC or Cr.sub.3 C.sub.2. As shown in FIG. 21, the wear resistance against water containing earth and sand considerably depends upon the hardness of the sprayed coating. It is preferred that a satisfactory effect can be obtained if the hardness is Hv 1000 or higher as described above. As can be understood from from FIG. 21, the conventional sprayed coating cannot provide satisfactory hardness and wear resistance against water containing earth and sand.
In general, spraying is an operation in which not only hard particles such as WC or Cr.sub.3 C.sub.2 are heated and sprayed but also metal particles such as nickel, chromium or cobalt particles are mixed and simultaneously heated and sprayed. As a result, WC particles or Cr.sub.3 C.sub.2 particles are bound by molten metal of nickel, chromium or cobalt to form a film. The reason why the sprayed coating is lower in hardness than that of the sintered article is not that the WC or Cr.sub.3 C.sub.2 particles are not of a lower hardness but that the binder metal for binding the WC or Cr.sub.3 C.sub.2 particles has detects such as blow holes or that the binding strength between the binder metal and the WC or Cr.sub.3 C.sub.2 particles is not fully high.
In order to enlarge the binding strength between WC or Cr.sub.3 C.sub.2 particles, the spraying method has been improved variously. For example, a high speed flame spraying method has been proposed for increasing the spraying velocity of the particles by utilizing the combustion energy of a combustible gas and a detonation spraying method utilizing a detonation of a combustible gas. As a result, the particle velocity at the time of spraying can be increased as compared with the speed realized when the conventional plasma sprayed coating is formed, resulting in a sprayed coating of higher hardness. However, the wear resistance against sediment water containing earth and sand is insufficient even if the sprayed coating is formed by any of the aforesaid spraying methods. It has been understood from the element test shown in FIG. 19 that the reason for this resides in the fact the hardness of the sprayed coating is lower.
The foregoing conventional ceramic bearings and those disclosed in JP-A-60-81517 and JP-A-60-88215 are not technologies that have been successful when taking into consideration the difficulty in the sintering technology, deterioration in the reliability and unsatisfactory assembling facility due to an increase in the weight caused from increasing the diameter of the bearing and the sleeve.
The non-water supply drainage pump using a combination of the WC sprayed coating and the ceramic bearing (sintered article) ("Trybologist", No. 2, vol. 36) and the bearing structure for a hydraulic turbine that uses a WC sprayed coating and Cr.sub.3 C.sub.2 sprayed coating (resumes of the twenty-sixth lecture meeting titled "Turbo Machines") are not successfully while considering the hardness factor, the wear resistance and the reliability of the sprayed coating, with the main component being WC or Cr.sub.3 C.sub.2.
The proposed improved methods for properties of the sprayed coating according to, for example, JP-A-57-2872 and JP-A-60-149762, encounter a limitation in coating materials for use therein. As the coating material, those having a thermal expansion coefficient which approximates that of ferrous alloys, which are primary base materials, or these which are not decomposed by the improving heat treatment, are used. The sprayed coating, the main component of which is WC or Cr.sub.3 C.sub.2, and from which satisfactory hardness can be attained, results in a large difference in the thermal expansion with respect to the base material. As a result, the coating is ruptured and it cannot be used as it is. Furthermore, an apparatus such as a pump that is used in water must be mainly made of stainless steel. Therefore, according to the conventional heat treatment of improving properties of the sprayed coating, the heat treatment temperature is higher than the annealing temperature of a stainless steel as a base material or the temperature at which the stainless steel is sensitized to intergranular corrosion. Hence, the hardness and the wear resistance of the base material deteriorate, causing a problem to arise in that the reliability deteriorates.
That is, the prior art has not been taken into consideration any improvement in properties of the sprayed coating, the main component of which is WC that has a large difference in the thermal expansion coefficient from ferrous alloys. Moreover, no consideration has been given with regard to the hardness of the base material that wear resistance.
The prior art disclosed in JP-A-55-90718 and having the arrangement that external water is supplied to the sliding portion of the bearing results in a problem of reliability because the operation of the drainage facility is stopped due to a malfunction of an external water supply apparatus and the drainage operation can be stopped in an emergency.